Electroplating of various metals with chromium is widely used in industry. There were hitherto generally distinguished two different types of electrodeposited platings of chromium, namely:
a. Bright chromium which is a decorative and anti-corrosive plating; PA1 b. Hard chromium which serves as wear-resistant layer which increases the useful time of life of many important machine parts. PA1 a. A plating bath prepared from chromium trioxide as main constituent and Cl as second component, defined as "Chromispel C". PA1 b. A plating bath prepared from chromium trioxide as main constituent and "I" (iodine and/or iodide) as second component, defined as "Chromispel I". PA1 c. A plating bath containing chromium trioxide as main constituent and Cl+I as further components, defined as "Chromispel-CI". PA1 d. Plating baths according to the above, which also contain a small quantity of sulfate ions (in the order of about 0.5 to about 2 percent by weight).
Whereas the thickness of bright chromium rarely exceeds 1 micron, platings of hard chromium may be of a thickness of the order of up to some hundreds of microns and sometimes even of some millimeters. Hard plating by means of chromium is used sometimes to restore worn-out machine parts, such as parts of ship-engines and others.
The bath which will be refered to later on as a conventional one is meant to be based principally on the U.S. Pat. No. 1,581,188 (1926) and/or on British Pat. No. 237288 (1925), including their further improvements. In the conventional bath for chrome electroplating the main constituent is chromium trioxide which is generally used in combination with sulfuric acid which serves as a catalyst. The conventional process for chrome plating has certain attractive features, such as a stable bath which is easily operated; the quality of the deposited chrome is generally high both in cases of bright chrome and hard chrome platings which however are to be obtained by means of two different versions of the conventional process.
The main drawback of the conventional process is its very low overall efficiency. Cathode current efficiency under industrial conditions rarely exceeds about 13 to 15%, while under laboratory conditions it may be up to about 20 to 25%. Thus only about 12 to 25% of the electrical energy consumed is actually utilized for the deposition of the metallic chromium while the remainder of the energy is wasted. This results also in considerable waste of time. For example, at a typical current density of about 40 A/dm.sup.2 a thickness of about 20 to 25 microns may be obtained with the conventional process during about 1 hour. Thus for a layer of a thickness of about 500 microns one needs, with the conventional process, about 20-25 hours.
Besides the conventional bath which is presently most widely used in industry, some alternative plating baths are also used, among which the most well known is the "Self Regulating Speed Chrome Bath" described in 1950 by Stareck, Parsal and Mahlstedt (Proc. Amer. Electroplat. Soc., vol. 37, p. 31). This process enables one to obtain a cathode current efficiency up to 22-24% which is higher than with the conventional process but still very low. Moreover this process has additional drawbacks, and the maintenance of stable properties of chrome plates during this process is difficult under industrial conditions.
It has been reported in literature that certain ions, such as F.sup.-, Cl.sup.-, SiF.sub.6.sup.- etc., can be used instead of SO.sub.4.sup.2- as catalysts. With regards to those other catalysts there are no adequate reports (except for F.sup.- and SiF.sub.6) relating to the effect of these ions on efficiency of the process and on properties of the chrome plating thus obtained. It may be stated that hitherto no replacement for SO.sub.4.sup.2- has been found.
There were described certain baths for electrodeposition of chromium using as a main constituent trivalent chromium rather than hexavalent one.
In certain cases those baths contained, besides trivalent chromium, also chlorine and several other additives as well (for example--see U.S. Pat. Nos. 3,706,636, 3,706,638 and 3,706,642) describing the use of carboxylic acid and glycolic acid together with chlorine (in trivalent chromium baths). These baths are very complicated and actually do not display advantages over the conventional baths.
There were also suggested Cl-containing baths using a non-aqueous solvent, as for example dimethylformamide (see for example J. Matulis et al., Lit. SSR Mokslu Akad. Darb. B1972(4) 34-40). The use of a non-aqueous solvent makes this method actually unsuitable for a wide industrial use. Moreover, the efficiency of the process is still low (about 30%).
There were described also baths based on a use of chromium chloride (in which chromium is trivalent), but not containing or almost not containing hexavalent chromium (see for example Brit. Pat. Appl. Ser. No. 25984/73).
Besides CrCl.sub.3 the bath contains NaCl, H.sub.3 BO.sub.3 and dimethylformamide. This bath is complicated and moreover does not display any considerable advantages as compared with the conventional one.
Also all the baths described based on a use of F.sup.- cannot in any way compete with the conventional bath.
There exist also reports describing a deterioration of chrome plating baths due to a presence of chlorine. These researches have been performed under operating conditions irrelevant to those ensuring the production of good quality chrome platings, as will be demonstrated later.
Certain more "exotic" baths for chromium electroplating have been suggested as well, for instance, the baths using perchlorates and pulsating currents etc.
However, a bath which could be considered as a substitute for the conventional one, i.e. ensuring the preparation of good quality chrome platings, with simultaneous increase of the current efficiency of the process and being easily operated under industrial conditions, has not yet been found.
It is an object of the present invention to provide an improved process of chromium electroplating which overcomes the drawbacks of the processes known hitherto and results in a considerable improvement of the current efficiency of the process.